1. Field of Invention
The present invention relates to a method for forming a dielectric layer. More particularly, the present invention relates to a method for forming the dielectric layer of a dynamic random access memory (DRAM) capacitor capable of storing a greater amount of electric charges.
2. Description of Related Art
As microprocessors become more powerful, the amount of memory necessary to carry out software programming increases correspondingly. Hence, memory units having a capacitor with a high capacitance are in great demand. There are several means of increasing the charge storage capacity of a capacitor. A high dielectric constant material may be selected to form the dielectric layer of a capacitor so that more electric charges can be stored per unit area of the capacitor. A second method is to reduce the thickness of the dielectric layer. However, quality of the dielectric material is likely to set a limit on the minimum thickness a dielectric layer may have. Alternatively, one may try to increase the effective surface area of a capacitor so that the total number of electric charges stored within the capacitor is increased.
As level of device integration continues to increase, surface area for constructing a DRAM capacitor actually shrinks. Means of increasing charge storage capacity of a capacitor falls on choosing a high dielectric constant material. Since tantalum pentoxide (Ta.sub.2 O.sub.5) has a high dielectric constant, it is one of the ideal materials for forming the dielectric layer of a capacitor. In general, a capacitor is formed by forming a polysilicon lower electrode over a substrate. A dielectric layer, preferably a tantalum pentoxide, is next deposited over the polysilicon electrode. The final step is to form a titanium nitride layer and a second polysilicon layer sequentially above the dielectric layer. The titanium nitride layer and the second polysilicon layer together form an upper electrode. To lower the amount of leakage current from the capacitor, a high-temperature treatment is usually carried out after the dielectric layer is formed. The high-temperature treatment is typically conducted in the presence of oxygen. However, oxygen is able to penetrate through the tantalum pentoxide layer and react with silicon in the lower polysilicon electrode. Therefore, a silicon oxide (SiO.sub.2) layer is formed at the interface between the tantalum pentoxide layer and the lower electrode. Since the dielectric constant of silicon dioxide is rather low, capacitance or storage capacity of the capacitor is lowered.
Furthermore, a depletion region is formed next to the lower electrode resulting from an electric field generated by applied voltages. The presence of a depletion region in the capacitor reduces the capacitance of the capacitor even further.